Generally, a backlight unit includes components, such as a light source and a light guide plate for guiding light from the light source as a planar light, housed between a lower frame and an upper frame, and the upper frame and the lower frame are engaged with each other. Such an engaging portion is described, for example, in JP 2006-285161 A, where a combination of a buckle element integrated with a part of a side surface of the upper frame and a corresponding projecting element integrated with a part of a side surface of the lower frame, and a combination of a T-shaped hook integrated with a part of a side surface of the upper frame and a groove formed in a part of a side surface of the lower frame.